1. Field of the Invention
The present invention relates to a control of an automatic transmission in a vehicle, and more particularly, to a control of a friction engaging means such as a brake and a clutch in the speed stage shifting of an automatic transmission having a tandem connection of speed stage change-over gear units.
2. Description of the Prior Art
It is well known in the art of the automatic transmission in a vehicle to construct the gear train of the automatic transmission by a tandem connection of a first and a second gear unit each including an assembly of gear wheels such as the planetary gear mechanism and friction engaging means such as clutches and brakes so that selective engagement and disengagement of the friction engaging means in each gear unit selectively sets up certain reduction gear ratios in each gear unit while the automatic transmission provides certain overall reduction gear ratios corresponding to the multiplication of the gear ratios in said first and second gear units.
In certain such automatic transmissions, including, for example, the one disclosed in U.S. Pat. No. 5,113,725 scheduled to be assigned to the same assignee as the present application, the first gear unit is changed over between a lower gear stage having a larger reduction gear ratio and a higher gear stage having a smaller reduction gear ratio, while the second gear unit is changed over among three successively raised gear stages having successively decreasing reduction gear ratios, respectively, wherein the lower and higher gear stages of the first gear unit are combined with each of the three gear stages of the second gear unit so that six successively raised gear stages are available to provide six successively decreasing reduction gear ratios.
In such an automatic transmission, a relative difference in the timing of engagement or disengagement of the friction engaging means in the first and second gear units in the changing-over of the gear stages that would generally have a certain fixed tendency such that the changing-over process of the first gear unit always proceeds relatively earlier or later than that of the second gear unit, unless particularly controlled in this respect, could cause greater instability in the gear stage shiftings, because when the first and second gear units are changed over in the same direction (from a lower gear stage to a higher gear stage or vice versa), the above-mentioned relative difference in the speed of progress of the changing-over process inherent to each product due to design or allowance in manufacture is suppressed by interference of operation of each other, while when the first and second gear units are changed over in mutually reversed directions (as in the shifting from the 2nd speed stage to the 3rd speed stage in the embodiment described hereinunder), the above-mentioned relative difference is amplified by compensation of operation of each other.